Various techniques have been used over the years to determine the coagulation status of blood. The standard in medical technology for some time was the fibrometer. Recently devices have been introduced that detect changes in the coagulation status of blood or plasma indirectly by measuring changes in electrical characteristics of the fluid with time.
A fibrometer is a device with a heat block to condition the temperature of a sample to be tested, a timer, and a mechanical probe that moves in the sample to detect when a clot has formed. A couple of drops of citrate anti-coagulated plasma is held in the heat block with the probe positioned above. At the instant a technician injects a coagulant into the plasma he presses a button that starts a timer and drops the moving probe down into the plasma. The timer stops at the time when the plasma becomes coagulated enough to stop the probe from moving. Of course, one can see that precision of this assay can vary significantly depending on the skill of the technician. In addition the sample size can be excessive, especially considering the confirmatory retesting typically required.
In U.S. Pat. No. 7,144,495, to Teodorczyk, the viscosity of a fluid is detected as changes in DC current flowing through the fluid with time. For example, an electric potential is applied to an electrochemical cell containing the fluid to first achieve a steady state cell current. A decrease in the steady state cell current is then detected and related to a change in viscosity of the sample. However, the method can be problematic because the fluid contact electrodes can be expensive in materials and construction. Further, use of DC current can exhaust current carriers in the sample, produce excessive heat, and generate gasses by electrolysis. These problems can affect the precision and consistency of the assay.
In U.S. Pat. No. 6,673,622, to Jina, an AC current is applied to a sample through contact with electrodes in a sample chamber. The viscosity of the sample is evaluated according to the impedance of the sample. Although this avoids some problems associated with DC current in Teodorczyk, the devices of Jina still require expensive electrode conductors, electrode contact with the sample, and careful sample handling for reproducible results.
In view of the above, a need exists for methods of coagulation analysis that do not require electrode contact with the sample. It would be desirable to measure sample viscosity over time without the assay itself influencing the sample. Benefits could also be realized through methods and devices that are less sensitive to variances in sample loading. The present invention provides these and other features that will be apparent upon review of the following.